1. Field of the Invention
The present invention relates in general to an optical disc apparatus, and more particularly to a tilt detection method and tilt detector which can detect a tilt angle of an optical axis of a beam of light projected on a recording medium.
2. Description of the Prior Art
In an optical disc apparatus, a beam of laser light emitted from a light source must be projected perpendicularly to a recording surface of an optical disc to accurately record or reproduce information thereon or therefrom. However, in the case where the recording surface of the optical disc is bent or distorted, it is tilted relative to an optical axis of the beam of laser light emitted from the light source. Namely, the beam of laser light emitted from the light source is projected on the recording surface of the optical disc under the condition that it is tilted relative to the recording surface. Provided that the recording surface of the optical disc is tilted, the beam of laser light will travel on the recording surface of the disc at a distance varying with a position, resulting in a complexity in the shape of a beam splitter and an inaccurate reading of information to be reproduced.
A description will hereinafter be given of conventional tilt detectors capable of overcoming the above problem, with reference to the accompanying drawings. FIG. 7 is a view schematically showing the construction of a conventional radial tilt detector for detecting a radial tilt amount, or a tilt amount in a radial direction of an optical disc 1. In FIG. 7a, a photodetector 3 is shown to include light receivers 31 and 32 for outputting detected results corresponding to the amounts of received light, respectively. Because the optical disc 1 is kept deformed due to a bending or distortion, a recording surface of the disc 1 is tilted relative to an optical axis of a beam of light emitted from a light source if the disc 1 is fixed to a spindle 2 in a tilted state, which is an output shaft of a spindle motor 4. For a tilt rising at the center of the optical disc 1 in a radial direction of the disc 1 and falling at the outer edge of the disc 1 in the radial direction as shown in FIG. 7a, a beam of light arriving at a light receiving surface of the photodetector 3 has a deformed elliptical shape as indicated by the reference numeral 35 in FIG. 7b. 
For a tilt falling at the center of the optical disc 1 in the radial direction of the disc 1 and rising at the outer edge of the disc 1 in the radial direction as shown in FIG. 7c, a beam of light arriving at the light receiving surface of the photodetector 3 has a deformed elliptical shape as indicated by the reference numeral 36 in FIG. 7d. If a beam of light is projected and reflected on the recording surface of the optical disc 1 under the condition that the disc 1 is tilted as above stated, a beam of light arriving at the light receiving surface of the photodetector 3 has a deformed elliptical shape having a coma-aberration, not a precise to circular shape.
In the conventional radial tilt detector, as shown in FIGS. 7a and 7b, a beam of light for detection of a tilt amount is emitted from a light source 30 to the optical disc 1 separately from a beam of light for information reproduction. The beam of light emitted to the optical disc 1 is reflected therefrom and received by the light receivers 31 and 32, into which the photodetector 3 is divided in two perpendicularly to the radial direction of the disc 1. A radial tilt amount of the optical disc, or a tilt amount in the radial direction of the optical disc, relative to an optical axis of the light beam emitted from the light source 30 is detected from a difference between detection signals from the light receivers 31 and 32.
FIG. 8 is a view schematically showing the construction of a conventional tangential tilt detector for detecting a tangential tilt amount, or a tilt amount in a tangential direction of the optical disc 1. In FIG. 8a, a photodetector is shown to include light receivers 51 and 52 for outputting detected results corresponding to the amounts of received light, respectively. For a tilt rising at the front portion of the optical disc 1 in a tangential direction of the disc 1 and falling at the rear of the disc 1 in the tangential direction as shown in FIG. 8a, a beam of light arriving at a light receiving surface of the photodetector 5 has a deformed elliptical shape as indicated by the reference numeral 55 in FIG. 8b. For a tilt falling at the front portion of the optical disc 1 in the tangential direction of the disc 1 and rising at the rear portion of the disc 1 in the tangential direction as shown in FIG. 8c, a beam of light arriving at the light receiving surface of the photodetector 5 has a deformed elliptical shape as indicated by the reference numeral 56 in FIG. 8d. 
In the conventional tangential tilt detector, as shown in FIGS. 8a and 8b, a beam of light for detection of a tilt amount is emitted from a light source 50 to the optical disc 1 separately from a beam of light for information reproduction. The beam of light emitted to the optical disc 1 is reflected therefrom and received by the light receivers 51 and 52, into which the photodetector 5 is divided in two perpendicularly to the tangential direction of the disc 1. A tangential tilt amount of the optical disc, or a tilt amount in the tangential direction of the optical disc, relative to an optical axis of the light beam emitted from the light source 50 is detected from a difference between detection signals from the light receivers 51 and 52 (the light emitting unit emitting the beam of light for detection of the tilt amount and the two light receivers will collectively be referred to hereinafter as a tilt sensor).
In the above-mentioned method, however, an optical pickup must comprise two tilt sensors, a tangential tilt sensor for detecting a tangential tilt amount and a radial tilt sensor for detecting a radial tilt amount, in addition to a photodetector for receiving a beam of light reflected from a recording medium and outputting an information reproducing signal in response to an optical signal of the received light beam. For this reason, the optical pickup is increased in size and cost and inconvenient to install.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a tilt detection method and tilt detector which can make the size of an optical pickup small, improve an installation efficiency of the pickup and reduce a manufacturing cost of the pickup.
In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a tilt detection method for a tilt detector for projecting a beam of laser light from a light source on a recording medium of a disc shape and detecting a tilt of an optical axis of the projected beam of light relative to the recording medium, the method comprising the step of detecting a tilt in a tangential direction of the recording medium, delaying the detected result for a predetermined period of time corresponding to the rotation of the medium by 90xc2x0 or 217xc2x0 and outputting the delayed result as a tilt in a radial direction of the medium.
In accordance with another aspect of the present invention, there is provided a tilt detection method for a tilt detector for projecting a beam of laser light from a light source on a recording medium of a disc shape and detecting a tilt of an optical axis of the projected beam of light relative to the recording medium, the method comprising the step of detecting a tilt in a radial direction of the recording medium, delaying the detected result for a predetermined period of time corresponding to the rotation of the medium by 90xc2x0 or 270xc2x0 and outputting the delayed result as a tilt in a tangential direction of the medium.
In accordance with a further aspect of the present invention, there is provided a tilt detector for projecting a beam of laser light from a light source on a recording medium of a disc shape and detecting a radial tilt of an optical axis of the projected beam of light relative to the recording medium, comprising delay means for delaying the detected result of the radial tilt for a predetermined period of time corresponding to the rotation of the recording medium by 90xc2x0 or 270xc2x0 and outputting the delayed result as a tilt in a tangential direction of the recording medium.
In accordance with yet another aspect of the present invention, there is provided a tilt detector for projecting a beam of laser light from a light source on a recording medium of a disc shape and detecting a tangential tilt of an optical axis of the projected beam of light relative to the recording medium, comprising delay means for delaying the detected result of the tangential tilt for a predetermined period of time corresponding to the rotation of the recording medium by 900 or 270xc2x0 and outputting the delayed result as a tilt in a radial direction of the recording medium.